1. Field of the Invention
The present invention generally relates to an image forming apparatus which transfers a toner image formed on a surface of a photosensitive body onto a recording medium at high accuracy by pressing the recording medium on the surface of the photosensitive body.
2. Description of the Related Art
Conventionally, in many cases, a continuous paper printer has been used for printing outputs from a computer on a ledger sheet. Recently, by utilizing a function which prints variable information at a high speed, the continuous paper printer has been used for printing various documents such as direct mail, invoices, instruction manuals, and books. By widened application of the continuous paper printer, the continuous paper printer must use various kinds of the recording media, for example, thin paper, thick paper, high-quality paper, and rough surface paper.
In addition, from a viewpoint of resource savings, both sides printing has been widely used. When both sides printing is executed by a cut sheet printer, an image is printed on the first surface (front surface) of a sheet, and another image is printed on the second surface (back surface) of the sheet by reversing the sheet.
In addition, in some cases, both sides printing is executed by disposing two continuous paper printers in tandem. In this case, a first continuous paper printer prints an image on the first surface of a recording medium and a second continuous paper printer prints another image on the second surface of the recording medium by reversing the recording medium. In order to obtain a high-quality image, image transfer performance of transfer sections in the continuous paper printer must be high.
The printer (image forming apparatus) includes a fixing section for fixing a toner image transferred onto a recording medium. The fixing section is disposed at a downstream side of the transfer section, and includes a pre-heater for preheating the recording medium by contacting the recording medium, a heat roller having a heater, and a backup roller for applying pressure to the heat roller.
In the fixing section, the pre-heater and the heat roller continue to apply heat to the recording medium while executing continuous printing and stop applying the heat when the continuous printing stops. Soon after the continuous printing stops, since the carrying speed of the recording medium does not immediately respond to the heat amount being applied to the recording medium, the recording medium is over-heated.
Consequently, a part of the recording medium stopped on the pre-heater may be expanded or shrunk or may have a concave-convex surface formed by the heat. In both sides printing, when a part of the recording medium, which part is stopped on the pre-heater in the first printer or in the first printing of a printer, is passed through the second printer or the second printing of the printer, a part of the toner image on the surface of the photosensitive body may not be transferred onto the recording medium due to the rough surface of the recording medium caused by the heat. That is, a high-quality image may not be printed due to the toner image non-transferred part.
In addition, in both sides printing, in the second printer or the second printing in a printer, the contact of the recording medium with the photosensitive body may be reduced due to damage (expansion or shrinkage) on the recording medium caused by the heat in the first printer or the first printing in the printer.
In Japanese Laid-Open Patent Application No. 9-171308, a recording medium is pressed on a photosensitive body by using a transfer assist blade, and a toner image is transferred onto the surface of the recording medium by correcting the rough surface of the recording medium with the transfer assist blade. With this, image quality degradation on the surface of the recording medium caused by fatigue and/or deformation of the surface of the recording medium due to the heat application is reduced.
In addition, various kinds of recording media are used in the continuous paper printer, and the width of the recording medium may be changed corresponding to application. When a wide photosensitive body of width 21 to 22 inches (533.4 to 558.8 mm) is used, the recording media whose widths are 12, 16, and 22 inches can be used. For example, when the recording medium is a ledger sheet, the width of the ledger sheet is 12 or 16 inches. When an instruction manual or a book is printed, since the printed recording medium is cut after the printing, the width of the photosensitive body is 17inches (431.8 mm) from which two sheets of A4 size (210 mm×297 mm) recording medium are obtained. Further, when a photosensitive body of 22-inch (558.8 mm) width is used, three sheets of B5 size (182 mm×257 mm) recording medium can be obtained.
Therefore, in a case where an image is printed on a recording medium of short width, when a transfer assist blade whose width is greater than the width of the recording medium is used, a part of the transfer assist blade sticks out from the recording medium and the stuck out transfer assist blade may damage the photosensitive drum.
In order to solve the above problem, in Japanese Laid-Open Patent Application No. 9-171308, plural assist blades are used and the plural assists blades are combined corresponding to the width of the recording medium.
In Japanese Laid-Open Patent Application No. 9-171308, the combination of the plural transfer assist blades responds to a recording medium of an arbitrary width; however, each of the plural transfer assist blades is independently driven by plural solenoids. Therefore, when the plural transfer assist blades are disposed between the transfer section and the photosensitive body, the size of the image forming apparatus becomes large and the cost of the image forming apparatus becomes high.